1. Field of the Invention
The present invention relates to a wiring, a method for forming the wiring, a semiconductor device, and a method for manufacturing the semiconductor device.
Note that in this specification, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics, and an electro-optical device, a semiconductor circuit, an electronic device, and the like are all semiconductor devices.
Further, the present invention relates to an object, a method, a manufacturing method, a process, a machine, manufacture, or a composition of matter. The present invention relates to, for example, a semiconductor device, a display device, a liquid crystal display device, a light-emitting device, a driving method thereof, or a production method thereof. In particular, the present invention relates to, for example, a display device, a light-emitting device, or a driving method thereof.
2. Description of the Related Art
A technique for forming a transistor by using a semiconductor film formed over a substrate having an insulating surface has attracted attention. The transistor is applied to a wide range of semiconductor devices such as an integrated circuit and a display device. A silicon-based semiconductor film is known as a semiconductor film applicable to a transistor.
As a silicon-based semiconductor film which is used as a semiconductor film of a transistor, either an amorphous silicon film or a polycrystalline silicon film is used depending on the purpose. For example, in the case of a transistor included in a large-sized display device, it is preferred to use an amorphous silicon film, which can be formed using the established technique for forming a film on a large-sized substrate. On the other hand, in the case of a transistor included in a high-performance display device where driver circuits are formed over the same substrate, it is preferred to use a polycrystalline silicon film, which can form a transistor having a high field-effect mobility. As a method for forming a polycrystalline silicon film, high-temperature heat treatment or laser light treatment which is performed on an amorphous silicon film has been known.
Further, in recent years, a transistor including an oxide semiconductor film has attracted attention. For example, a transistor which includes an amorphous oxide film containing indium, gallium, and zinc and having a carrier density lower than 1018 cm−3 is disclosed (see Patent Document 1).
An oxide semiconductor film can be formed by a sputtering method or the like and therefore can be applied to a transistor included in a large-sized display device. Moreover, a transistor including an oxide semiconductor film has a high field-effect mobility; therefore, a high-definition display device or a high-performance display device where driver circuits are formed over the same substrate can be obtained. In addition, there is an advantage that capital investment can be reduced because part of production equipment for a transistor including an amorphous silicon film can be retrofitted and utilized.
When the size of a display device is increased, the lengths of wirings are increased. Further, when the definition of a display device is increased, wirings need to be reduced in width and thickness accordingly. Thus, an increase in size and/or definition of a display device leads to an increase in wiring resistance. In other words, an increase in size and/or definition of a display device might cause a problem such as signal delay (also referred to as wiring delay) due to wiring resistance.
In order to reduce wiring resistance, use of a conductive film containing copper, which has low resistance, for wirings has been proposed. However, methods for etching a conductive film containing copper are mostly etching using a chemical solution, which is isotropic etching. Therefore, it has been difficult to control the shape of the conductive film containing copper after the etching. In the case where the conductive film containing copper has a large taper angle, for example, a shape defect of a layer over the conductive film containing copper might be caused.